1. Field of the Invention
The present invention relates to aqueous dispersions based on poly(meth)acrylates, produced in a multi-state emulsion polymerization process, and to the use of such dispersions to prepare adhesives (bonding compositions) which are susceptible to hot-sealing.
2. Description of the Background
In the past, hot-sealing compositions have been devised for the packaging industry, which compositions fully meet the applicable technical criteria, but which contain substantial amounts of organic solvents. E.g., Eur. Pat. 129,178 describes a hot-sealable coating composition comprised of a film-forming dispersion of at least two different polymer types with different adhesion properties in an organic solvent system, wherewith the dispersion contains polymers of the two types and in addition contains a polymer comprised of units corresponding to the two polymer types. Vinylidene chloride polymers have also been used widely in the past as hot-sealable compositions.
Coating compositions for sealing metal substrates are of particular practical and economic importance; in particular, compositions for providing seals between aluminum and plastics such as polystyrene, polypropylene, and polyvinyl chloride, or between aluminum and glass. (See Ullmanns Enzyklopaedie der techn. Chemie, 6th Ed., 1985 ff., Vol 16, pub. Verlag Chemie, pp. 101-103.)
Ger. Pat 4,209,651 (to BASF AG) describes an aqueous dispersion containing a copolymer A in the presence of which a polymerization is carried out to produce a copolymer B. Copolymer A is produced by bulk polymerization, and is not a core-and-shell product. The product materials are used in sealing PVC to paper board.
Jap. Pat. 08-231729 A (Derwent No. 96-461419/46). The products comprise core-and-shell polymers produced by emulsion polymerization followed by sprav drying. The shell has larger amounts of carboxyl-group-containing monomers which improve the re-dispersibility of the product in water.
Rising awareness of environinental issues has created a trend away from solvent-containing formulations and toward aqueous dispersions such that when, e.g., a coating or adhesive is applied to a surface, organic compounds are not emitted into the environment. Also significant is the trend away from chlorine-containing products, and, perhaps to a lesser extent, aromatic compounds.
Technology has been able to adjust, to some extent, to these more stringent requirements. Thus, as disclosed in Ger. 29 06 118, polymers of vinylidene chloride, used as hot sealing adhesives, may be applied in the form of aqueous dispersions. In order to improve adhesion to un-primed plastic and metal sheets, special "anchoring agents" may be added to the composition. Suitable such additives are water-soluble polymers of functionally substituted acrylamides and/or methacrylamides, or copolymers of these monomers with acrylamide and/or methacrylamide, which (co)polymers are present as species dissolved in the aqueous phase of the dispersion.
Two methods of coating of aluminum foils and sheets are in widespread use:
In the case of relatively aggressive filler materials, first a primer coat of about 2 microns in thickness is applied to the aluminum foil. This primer is comprised of a vinyl-chloride-containing solution polymer. A finish coating comprised of a methacrylate resin of 4-8 micron thickness is then provided over the primer coat.
For cases of less aggressive filler materials, a single-coat system is used, employing a hot-sealable coating of thickness of about 5-10 microns comprising a mixture of the abovementioned polymers.
Still unsolved is the problem of providing a non-chlorinated, hot-sealing adhesive in the form of an aqueous dispersion with adequate adhesion. Ger. 27 27 914 (U.S. Pat. No. 4,291,090) and Ger. 28 55 147 disclose hot-sealing adhesives based on polyacrylate which can be applied as aqueous dispersions. In some cases, these adhesives are not comprised exclusively of esters of (meth)acrylic acid. That is, they may contain one or more comonomers, employed, e.g., for the purpose of modifying the melting temperature, hardness, or adhesion properties of the adhesive. Suitable comonomers include, inter alia, (meth)acrylamide, and aminoalkyl esters of acrylic acid and/or of methacrylic acid. The specific effects which these comonomers have on the properties of the polymers is not disclosed.
Ger. 39 30 743 (Can. Pat. App. 2,025,368.1) describes an aqueous polyacrylate dispersion, which is useful as a hot-sealing adhesive, based on at least 70 wt. % of a polyacrylate comprised of units of lower alkyl methacrylates and also containing polymerized (meth)acrylamide or aminoalkyl (meth)acrylate. Other publications relate to aqueous dispersions of copolymers of olefins and/or vinyl acetate. which copolymers contain units of chlorinated monomers and are produced in a single-stage process. The described dispersions exhibit poor adhesion to aluminum substrates.
Polymer dispersions, which are prepared from polymer particles having a core-and-shell structure, which are described in the literature, either contain chlorinated monomers or are not suitable for bonding plastics to aluminum.
Eur. Pat. 574,803 describes aqueous polymer dispersions of 5-80 wt. % of a copolymer (A), having a glass transition temperature of 50-150.degree. C., and 95-20 wt. % of a copolymer (B), having a glass transition temperature of -50 to +50.degree. C., the two copolymers in the composition having with a temperature difference of 20.degree. C. There is no specific indication in this publication that the proposed products are suitable for bonding plastics to aluminum.
Further, no appropriate two-coat methods are described in the cited state of the art references.
A two-coat coating is proposed in Jap. Pat. App. 54-161,684 (Chem.Abstr. 92:199427x). First, an aluminum foil is coated with the aqueous dispersion of a monomer and then with a mixture of an ethylene-vinyl acetate copolymer, colophony, and wax, to a thickness of 20 microns.
In connection with the object of formulating chlorine-free aqueous polymer dispersions for use as hot-sealing adhesives, another problem exists which is the problem of compatibility with the state of the art apparatus employed. This problem is particularly significant in the large-scale coating of aluminum foils and sheets. In particular, one should be able to use customary methods of applying polymers as thin films (see Polymer Handbook, 2nd Ed., pub. Hanser Verlag), and the films employed should not be sticky at ordinary temperatures.
A requirement imposed on means of hot sealing an adhesive to a suitable substrate is that good adhesion for bonding the two materials (e.g. aluminum and polystyrene) must be achieved in a short time and at temperatures which are not excessive and which are about, e.g., 180-220.degree. C.
The proposed packaging methods are also intended for used with foodstuffs. In such applications, the monomers and the adjuvants must be selected such that the regulations governing food packaging (German Federal Health Office guidelines BGA 14, and the corresponding US FDA guidelines) are complied with.
In Ger. 39 30 743 an aqueous dispersion is proposed which has adequate adhesion to an aluminum foil substrate coated with a vinyl chloride copolymer as a primer. Efforts to modify such dispersions by incorporating units of adhesion-promoting monomers in order to provide a variant which can be applied to un-primed aluminum have not been successful. Thus the reference does not provide a solution to the problem of replacing chlorinated primers. Other efforts using similar approaches, using polymers with simple particle structures, have not been successful.